This invention relates to a process for expanding thermoformable materials and products in general and more particularly, to a process which permits having a clear surface on one side of the expanded material. In a series of patents granted to Walter Smarook and including U.S. Pat. Nos. 3,765,810 and 3,919,446 there is disclosed a process and apparatus for expanding thermoformable material in which a sheet of thermoformable material is disposed between two platens having a plurality of vented openings on the working surface. The platens are heated to a temperature of at least 70.degree. C. at which point the material becomes tacky and is bonded to the platen by hot tack adhesion. The platens are then moved apart expanding the material in such a manner that, at the locations of the vented openings voids are formed in the material. The process disclosed in these patents was limited to relatively small sizes of material. As defined in these patents, thermoformable means that the described material is solid at 25.degree. C. but can be reshaped or reformed above some higher temperature. Also defined in these patents is a temperature " Ta" which means a temperature at which a thermoplastic material exhibits hot tack adhesion.
U.S. Pat. No. 4,113,909 granted to Donald R. Beasly and assigned to the Norfield Corporation describes the manner in which such a process can be supplied to the formation of large sheets, e.g., 5.times.10 feet sheets of material. Such material has found numerous uses of an architectural and structural nature. Expanded panels can be made having voids on both sides and also with one flat side of the other side containing voids. However, the nature of the process has been such that when making a smooth sided expanded panel the smooth surface has an opaque appearance. It has been found desirable to have a smooth sided expanded panel that has a clear and smooth surface. There is, of course, many applications for clear or transparent panel of this nature such as in skylights, green houses, curtain walls, domes and so on.
Thus, the desirability of being able to make an expanded panel with a clear side for use in various architectural applications becomes evident.